Heart failure (HF) is an important clinical problem and abnormalities in Ca2+ handling are considered to be a significant contributor to cardiac dysfunction. Decreased activity of the Ca2+ ATPase of the sarcoplasmic reticulum (SERCA2) occurs in failing hearts. Increasing SERCA2 activity through SERCA2 transgene expression may, therefore, improve cardiac function. In Aim I, we will evaluate if pressure overload (PO) induced decreases in SERCA2 activity, abnormalities in Ca2+ handling, and contractile function can be prevented and compensated for in SERCA2 transgenic animals. PO-induced decreases in SERCA2 could only be obtained in rats and not in mice. We, therefore, produced SERCA2 transgenic rats which will be submitted to PO by aortic constriction (AC). We will determine in these SERCA2 rats if the PO- induced decrease in SERCA2 levels and activity and resultant abnormalities in Ca2+ handling and contractile function can be prevented. SERCA2 activity can also be increased by diminishing the inhibitory influence of unphosphorylated phospholamban (PLB). This approach will be pursued in Aim II by expressing mutant PLB with decreased SERCA2 interaction and by decreasing PLB levels using an anti- PLB ribozyme construct in isolated cardiac myocytes and in transgenic animals. Using an anti-SERCA2 ribozyme approach, SERCA2 levels by themselves can be lowered to determine the role of diminished SERCA2 in the induction of heart failure. In Aim III, we will explore if decreases in SERCA2 levels and other potential consequences of PO persisting for some time can subsequently be increased and result in improvement of Ca2+ handling and contractile performance. Hearts from rats with PO persisting for some time and developing heart failure will be used for viral vector mediated expression of SERCA2 and PLB related transgenes. SERCA2 activity, Ca2+ handling, and contractile function will be assessed in myocytes, papillary muscle strips from such hearts to determine if functional restoration can occur. Our findings will provide new knowledge related to the role which lowered SERCA2 activity plays in the development of HF and if maintaining or increasing SERCA2 activity can improve cardiac function and play a potential role in the treatment of HF.